Concrete

ABSTRACT

Concrete comprising water and cement in which the aggregates of sand and stones wholly or partially are substituted by crushed glass, the substantial part of the sand fraction being substituted either by glass of a grain size of 0-5 mm, by a non alkali reactive mineral with said grain size, or by a combination of said two types of fractions. The substantial part of the stone fraction is substituted by glass of a grain size 5-20 mm. The concrete also comprises silica dust, flyash or finely crushed slag if the finest fraction of glass (0-5 mm) is present in significant amounts in the concrete.

[0001] The present invention relates to a glass concrete, i.e. aconcrete in which the aggregates wholly or partially consist of crushedglass, providing a particularly decorative appearance, which isdesirable for facades etc.

BACKGROUND

[0002] The main elements of a concrete are cement, water, sand andstone. At production the sand and stone materials are divided in severalfractions with respect to grain size. In addition it is common to addchemical and mineral components to obtain the required properties forthe fresh and cured concrete.

[0003] “Glass concrete” is known per se, and has been used for certainpurposes where the aesthetical aspects have been a main consideration.The use of concrete with glass as aggregate is not however, free ofproblems, and it has not been possible to obtain a concrete that isnearly as good as concrete with conventional aggregates with respect tocompressive strength and fracture strength. Neither does the presentinvention give directions of a glass concrete that is particularly wellsuited for constructional use.

[0004] A particular problem associated with glass in concrete is thepresence of alkalies in the concrete and the thereby related problems.Since glass is alkali reactive and reacts with water soluble alkalies inthe concrete, expansion will usually occur, causing fissures in theconcrete. The problem is well known and attempts have been made to avoidit by using low alkali cement and different additives, but so far nosuccessful solution has been found.

[0005] Swedish patent No. 501 419 teaches the use of recycle glass as afiller in concrete. With filler in this context is meant a (glass)fraction with grain sizes from 0-0.25 mm. The characterizing part of thepatent is that 95% by weight of the glass filler used has a grain sizeless than 300 micrometer (0.3 mm). It is an object of the methodaccording to this patent to provide a concrete that has good compressivestrength and good properties with respect to workability of the freshconcrete. By using a filler with very fine grains it is possible toreduce the water content, which generally gives a better compressivestrength for the finished, cured concrete. As shall be furtherelaborated below, however, the choice of grain size according to thispatent implies a potential risk of alkali reactivity in the curedconcrete, especially in cases where the concrete also includes alkalireactive aggregates.

[0006] For the manufacture of alkali resistant concrete, it is importantthat the total content of alkali is as low as possible. It is thuscommon to use low alkali cement with equivalent Na₂O≦0.6%. In additionpossible alkali content in the aggregates or additives must be takeninto consideration.

OBJECTIVES

[0007] The object of the present invention is to provide a product thatis environmentally friendly and friendly with respect to the utilizationof resources, by utilizing recycle glass as an aggregate to concrete foraesthetical purposes. More particularly it is an object to provide aglass concrete with best possible properties with respect to compressivestrength and fracture strength. More particularly it is an object of theinvention to eliminate the probably most severe disadvantage of glassconcrete, namely the tendency of fissuration as a consequence of alkalireactions.

THE INVENTION

[0008] These objects are achieved by means of a concrete comprisingwater and cement in which the aggregate of sand and stones wholly orpartially is substituted by crushed glass, whereby the substantialamount of the sand fraction is substituted either by a glass fractionwith a grain size 0-5 mm, by a non alkali reactive mineral with saidgrain size or by a combination of said two fractions, whereas thesubstantial amount of the stone fraction is substituted by a glassfraction with grain size 5-20 mm, while the concrete additionallycomprises silica dust, flyash or finely crushed slag if the finestfraction of glass (0-5 mm) is present in significant amounts in theconcrete.

[0009] Preferred embodiments of the invention is disclosed by thedependent claims.

[0010] What has not really been given attention earlier, and for thefirst time is taken into consideration with the present invention, isthat the glass itself is an alkali source. This is illustrated throughlaboratory tests as referred below.

[0011] The laboratory tests were conducted the following way. Glass wascrushed to different grain sizes, extracted in a saturated solution ofcalcium hydroxide at 50° C. for up to 20 weeks. After predeterminedexposure times, samples were taken from each grain fraction and analysedwith respect to sodium and potassium. Total amount of released alkali asa function of grain size is shown below as g Na₂O equiv./100 g glass.TABLE 1 Extraction of alkali from glass. Fraction (mm) Na₂O/ 100 gFraction (mm) Na₂O/ 100 g glass    0-0.075 0.915 0.500-1.00  0.3430.075-0.125 0.870 1.00-2.00 0.229 0.125-0.250 0.826  2.00-10.00 0.0960.250-0.500 0.633 10.00-20.00 0.041

[0012] The results of table 1 indicates that releasable alkalies fromglass may contribute significantly to the alkali of the concrete. Itfurther shows that the amount extractable alkali per weight unit glass,increases with decreasing grain size. This observation forms part of thebasis for the present invention.

[0013] Tests were conducted with respect to alkali reactions accordingto Canadian Standard CSA A 23.2-14A. According to this method concreteprisms (10×10×45 cm) are exposed at 38° C. og 100% RH for up to oneyear. If the expansion of the prisms during the test period exceeds0.04%, the concrete is considered to be potentially alkali reactive.

[0014] 8 samples of concrete mixtures with recycle glass were tested formanufacture of glass concrete according to the invention. The mixturediagram and results of compressive strength in MPa after 28 days areshown in table 2. In the test program 3 types of cement were involved:Standard Portland cement (P30), HS65 from Norcem and White cement fromÅlborg Portland, Denmark. The two latter cement types are low alkalicements. To investigate the glass reactivity, one of the mixtures wasadded extra alkali in addition to the amount naturally occurring(mixture No. 6). The effect of the silica dust on the expansion (alkalireaction) was investigated with mixtures No. 5 and No. 7. Both thesemixtures were based on low alkali cements. In mixture No. 4 glass withgrain size 0-5 mm was substituted by a non alkali reactive mineral,granite. To investigate whether finely crushed glass exhibits pozzolaneproperties and thereby the same desirable effect on the alkalireactivity of the glass concrete as the silica dust, mixture No. 8 wasadded 74 kg glass powder/m³ concrete. TABLE 2 The composition of thetest mixtures (in kg/m3 concrete) and the results from tests of freshproperties and compressive strength Mixture No. 1 2 3 4 5 6 7 8 Na2equiv (of cement + 3.1 1.91 0.75 0.81 0.67 4.92 1.8 1.88 addition CementP30 365 366 HS65 374 352 368 White 357 384 317 Silica 35 35 Årdal sand769 Glass powder 74 Glass 0-5 mm 731 748 714 704 732 662 662 Glass 5-20mm 892 913 871 939 860 895 862 902 Defoamer 0.9 0.9 0.9 1 0.9 0.9 0.90.9 Na(OH)₂ 2.46 Water (tot) 210 194 223 202 233 211 247 223water/(cement + silica) 0.58 0.56 0.63 0.53 0.66 0.58 0.64 0.61 Sink, cm4 7 12 9 5 4 7 8 Density 2200 2245 2165 2295 2150 2210 2165 2230 Air, %3.4 1.7 3.6 1.7 3.4 2.8 2.2 1.2 Compressive strength 32.7 39.7 34.1 50.537.6 25.5 43.3 36.2 (MPa). 28 days

[0015] The results of 52 weeks aging at 38° C., 100% RH is shown intable 3 as % expansion (in length direction) of the original prismlengths (a “−” ahead the length change indicates a negative change, i.e.shrinking). TABLE 3 Length changes of glass concrete in % Time Mix. Mix.Mix. Mix. Mix. (weeks) Mix. 1 2 3 4 5 6 Mix. 7 Mix. 8 1 0.01 0 0.01 0 00.01 0 0 2 0.015 0 0.01 0 0 0.019 −0.01 0 4 0.037 0.01 0.01 0 0 0.041 00 8 0.13 0.058 0.01 0 0 0.139 0 0.024 12 0.228 0.157 0.028 0 0 0.261 00.09 16 0.36 0.294 0.091 0 0 0.396 0 0.181 26 0.699 0.67 0.447 0 0 0.7640 0.488 52 1.505 1.515 1.494 0 0 1.663 0 1.206

[0016] The results show that after 16 weeks or more only three glassconcrete mixtures (No-4, 5 and 7) satisfy the requirements to alkalireactivity according to CSA-A23.2-14A, i.e. that the expansion is lessthan 0.04%. These three glass concrete mixtures are based on low alkalicement with silica dust (mixture No. 5 and 7), alternatively a finefraction (grain size 0-5 mm) substituted by a non alkali reactivegranite (mixture No. 4). Normally use of low alkali cement will preventexpansion in a concrete produced with sand and stones respectivelycomprising alkali reactive components, but these tests show that lowalkali cement alone is not sufficient to prevent expansion of the glassconcrete (mixtures 2 and 3). A comparison of the results for mixture No.8 and with mixture No. 7 shows that glass dust does not have the samefavourable effect as silica dust regarding prevention of expansion ofglass concrete.

[0017] The results indicate that the production of alkali resistantglass concrete with crushed glass as the fine fraction, is conditionalon the addition of silica dust or the like. Flyash or finely crushedslag may be used as alternatives to silica dust. It is reason to believethat the unfavourable effect of crushed glass as a fine fraction is dueto an extraction of alkali from the glass that causes a high alkaliconcentration in the concrete mixtures, cf table 1.

[0018] As alkali extraction increases with decreasing grain size of theglass fraction, it may be beneficial to avoid completely glass withgrain size less than 0.5 mm, at least for purposes where there areparticular requirements to compressive strength and long timedurability. If for some reasons such a fraction is wanted in theconcrete, it should also be added a pozzolan (silica dust or the like),alternatively finely crushed material of non alkali reactive minerals.

1. Concrete comprising water and cement in which the aggregate of sandand stones wholly or partially is substituted by crushed glass,characterized in that the substantial part of the sand fraction issubstituted either by: glass of a grain size of 0-5 mm, a non alkalireactive mineral with said grain size, or by a combination of said twotypes of fractions, that the substantial part of the stone fraction issubstituted by glass of a grain size 5-20 mm, whereas the concrete alsocomprises silica dust, flyash or finely crushed slag if the finestfraction of glass (0-5 mm) is present in significant amounts in theconcrete.
 2. Concrete according to claim 1, characterized in that thefinest glass fraction has a grain size in the range 0.5-5 mm. 3.Concrete according to claim 2, characterized in that as the finestaggregate fraction and with a grain size typically less than 0.5 mm, aconventional pozzolane material or a finely crushed material of a nonalkali reactive mineral is used.